TAG_EXIST()

       To test whether a tag name exists in a structure.

       Routine obtains a list of tagnames and tests whether the requested one
       exists or not. The search is recursive so if any tag names in the
       structure are themselves structures the search drops down to that level.
       (However, see the keyword TOP_LEVEL).

       status = TAG_EXIST(str, tag, [ INDEX =, /TOP_LEVEL ] )

       str  -  structure variable to search
       tag  -  tag name to search for, scalar string

       Function returns 1b if tag name exists or 0b if it does not.

       TOP_LEVEL = If set, then only the top level of the structure is
                           searched.

       INDEX = index of matching tag, scalar longward, -1 if tag name does
               not exist

       Determine if the tag 'THICK' is in the !P system variable
       IDL> print,tag_exist(!P,'THICK')

       None.
 MODIFICATION HISTORY:     :
       Written,       C D Pike, RAL, 18-May-94
       Passed out index of matching tag,  D Zarro, ARC/GSFC, 27-Jan-95
       William Thompson, GSFC, 6 March 1996    Added keyword TOP_LEVEL
       Zarro, GSFC, 1 August 1996    Added call to help
       Converted to IDL V5.0   W. Landsman   September 1997
       Use SIZE(/TNAME) rather than DATATYPE()  W. Landsman  October 2001

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